1. Field of the Invention
The present invention relates to a copying machine having detectors for the color and density of the nonimage or background area of an original placed on the copy board.
2. Description of the Prior Art
In a conventional electrophotographic copying machine, many factors influence the copy quality of the copied image. The influence on copy quality caused by the density of the background of the original to be copied is particularly great and it has thus been necessary to manually adjust the exposure length or the developing bias voltage in accordance with the background density. However, such manual adjustment is extremely troublesome and copying efficiency is low; therefore, to automate this, an effective method wherein the density of background colors (i.e. the nonimage or non-information area) of the original is automatically detected and an appropriate developing bias voltage corresponding to the detected density is impressed on the developing device was studied.
As an example of such a copying machine, reference is made to Japanese Laid-Open Patent Publication No. 95030/1978. According to its disclosure, a light receiving element with a minute light receiving area is arranged in or near the path of the copying light of the copying machine and the detected output corresponding to the background density of the original is measured; the developing bias voltage is automatically controlled on the basis of the recognized one-to-one correspondence between said detected output and an appropriate developing bias voltage.
In such a detecting method for background density when the surface or background of the original is colored, said detected output produced from a light receiving element varies in accordance with both the color and the density and, as a consequence, reliance on a strict one-to-one correspondence with an appropriate developing bias voltage may be misplaced. Of further significance is the difference in the color sensitivity of the photosensitive body or member. These factors cause great inconvenience in that the kinds of originals with which an appropriate developing bias voltage can conventionally be properly selected are extremely limited.